Fungal infections of humans range from superficial conditions, usually caused by dermatophytes or Candida species, that affect the skin (such as dermatophytoses) to deeply invasive and often lethal infections (such as candidiasis and cryptococcosis). Pathogenic fungi occur worldwide, although particular species may predominate in certain geographic areas.
In the past 20 years, fungal infections have increased dramaticallyxe2x80x94along with the numbers of potentially invasive species. Indeed, fungal infections, once dismissed as a nuisance, have begun to spread so widely that they are becoming a major concern in hospitals and health departments. Fungal infections occur more frequently in people whose immune system is suppressed (because of organ transplantation, cancer chemotherapy, or the human immunodeficiency virus), who have been treated with broad-spectrum antibacterial agents, or who have been subject to invasive procedures (catheters and prosthetic devices, for example). Fungal infections are now important causes of morbidity and mortality of hospitalized patients: the frequency of invasive candidiasis has increased tenfold to become the fourth most common blood culture isolate (Pannuti et al. (1992) Cancer 69:2653). Invasive pulmonary aspergillosis is a leading cause of mortality in bone-marrow transplant recipients (Pannuti et al., supra), while Pneumocystis carinii pneumonia is the cause of death in many patients with acquired immunodeficiency syndrome in North America and Europe (Hughes (1991) Pediatr Infect. Dis J. 10:391). Many opportunistic fungal infections cannot be diagnosed by usual blood culture and must be treated empirically in severely immunocompromised patients (Walsh et al. (1991) Rev. Infect. Dis. 13:496).
The fungi responsible for life-threatening infections include Candida species (mainly Candida albicans, followed by Candida tropicalis), Aspergillus species, Cryptococcus neoforms, Histoplasma capsulatum, Coccidioides immitis, Pneumocystis carinii and some zygomycetes. Treatment of deeply invasive fungal infections has lagged behind bacterial chemotherapy.
There are numerous commentators who have speculated on this apparent neglect. See, for example, Georgopapadakou et al. (1994) Science 264:371. First, like mammalian cells, fungi are eukaryotes and thus agents that inhibit fungal protein, RNA, or DNA biosynthesis may do the same in the patient""s own cells, producing toxic side effects. Second, life-threatening fungal infections were thought, until recently, to be too infrequent to warrant aggressive research by the pharmaceutical industry. Other factors have included:
(i) Lack of drugs. A drug known as Amphotericin B has become the mainstay of therapy for fungal infection despite side effects so severe that the drug is known as xe2x80x9camphoterriblexe2x80x9d by patients. Only a few second-tier drugs exist.
(ii) Increasing resistance. Long-term treatment of oral candidiasis in AIDS patients has begun to breed species resistant to older anti-fungal drugs. Several other species of fungi have also begun to exhibit resistance.
(iii) A growing list of pathogens. Species of fungi that once posed no threat to humans are now being detected as a cause of disease in immune-deficient people. Even low-virulence baker""s yeast, found in the human mouth, has been found to cause infection in susceptible burn patients.
(iv) Lagging research. Because pathogenic fungi are difficult to culture, and because many of them do not reproduce sexually, microbiological and genetic research into the disease-causing organisms has lagged far behind research into other organisms.
In the past decade, however, more antifungal drugs have become available. Nevertheless, there are still major weaknesses in their spectra, potency, safety, and pharmacokinetic properties, and accordingly it is desirable to improve the the panel of anti-fungal agents available to the practioner.
The fungal cell wall is a structure that is both essential for the fungus and absent from mammalian cells, and consequently may be an ideal target for antifungal agents. Inhibitors of the biosynthesis of two important cell wall components, glucan and chitin, already exist. Polyoxins and the structurally related nikkomycins (both consist of a pyrimidine nucleoside linked to a peptide moiety) inhibit chitin synthase competitively, presumably acting as analogs of the substrate uridine diphosphate (UDP)-N-acetylglucosamine (chitin is an N-acetylglucosamine homopolymer), causing inhibition of septation and osmotic lysis. Unfortunately, the target of polyoxins and nikkomycins is in the inner leaflet of the plasma membrane; they are taken up by a dipeptide permease, and thus peptides in body fluids antagonize their transport.
In most fungi, glucans are the major components that strengthen the cell wall. The glucosyl units within these glucans are arranged as long coiling chains of xcex2-(1,3)-linked residues, with occasional sidechains that involve xcex2-(1,6) linages. Three xcex2-(1,3) chains running in parallel can associate to form a triple helix, and the aggregation of helicies produces a network of water-insoluble fibrils. Even in the chitin-rich filamentous aspergilli, xcex2-(1,3)-glucan is required to maintain the integrity and form of the cell wall (Kurtz et al. (1994) Antimicrob Agents Chemother 38:1408-1489), and, in P. carinii, it is important during the life cycle as a constituent of the cyst (ascus) wall (Nollstadt et al. (1994) Antimicrob Agents Chemother 38:2258-2265).
In a wide variety of fungi, xcex2-(1,3)-glucan is produced by a synthase composed of at least two subunits (Tkacz, J. S. (1992) In: Emerging Targets in Antibacterial and Antifungal Chemotherapy Sutcliffe and Georgopapadakou, Eds., pp495-523, Chapman and Hall; and Kang et al (1986) PNAS 83:5808-5812). One subunit is localized to the plasma membrane and is thought to be the catalytic subunit, while the second subunit binds GTP and associates with and activates the catalytic subunit (Mol et al. (1994) J Biol Chem 269:31267-31274).
Two groups of anticandidal antibiotics known in the art interfere with the formation of xcex2-(1,3)-glucan: the papulacandins and the echinocandins (Hector et al. (1993) Clin Microbiol Rev 6:1-21). However, many of the papulacandins are not active against a variety of Candida species, or other pathogenic fungi including aspergillus. The echinocandins, in addition to suffering from narrow activity spectrum, are not in wide use because of lack of bioavilability and toxicity.
Covalent modification by isoprenoid lipids (prenylation) contributes to membrane interactions and biological activities of a rapidly expanding group of proteins (see, for example, Maltese (1990) FASEB J 4:3319; and Glomset et al. (1990) Trends Biochem Sci 15:139). Either farnesyl (15-carbon) or geranylgeranyl (20-carbon) isoprenoids can be attached to specific proteins, with geranylgeranyl being the predominant isoprenoid found on proteins (Fransworth et al. (1990) Science 247:320).
Three enzymes have been described that catalyze protein prenylation: farnesyl-protein transferase (FPTase), geranylgeranyl-protein transferase type I (GGPTase-I), and geranylgeranyl-protein transferase type-II (GGPTase-II, also called Rab GGPTase). These enzymes are found in both yeast and mammalian cells (Schafer et al. (1992) Annu. Rev. Genet. 30:209-237). FPTase and GGPTase-I are xcex1/xcex2 heterodimeric enzymes that share a common xcex1 subunit; the xcex2 subunits are distinct but share approximately 30% amino acid similarity (Brown et al. (1993). Nature 366:14-15; Zhang et al. (1994). J. Biol. Chem. 269:3175-3180). GGPTase II has different xcex1 and xcex2 subunits and complexes with a third component (REP, Rab Escort Protein) that presents the protein substrate to the xcex1/xcex2 catalytic subunits. Each of these enzymes selectively uses farnesyl diphosphate or geranylgeranyl diphosphate as the isoprenoid donor and selectively recognizes the protein substrate. FPTase farnesylates CaaX-containing proteins that end with Ser, Met, Cys, Gln or Ala. GGPTase-I geranylgeranylates CaaX-containing proteins that end with Leu or Phe. For FPTase and GGPTase-I, CaaX tetrapeptides comprise the minimum region required for interaction of the protein substrate with the enzyme. GGPTase-II modifies XXCC and XCXC proteins; the interaction between GGPTase-II and its protein substrates is more complex, requiting protein sequences in addition to the C-terminal amino acids for recognition. The enzymological characterization of these three enzymes has demonstrated that it is possible to selectively inhibit one with little inhibitory effect on the others (Moores et al (1991) J. Biol. Chem. 266:17438).
GGPTase I transfers the prenyl group from geranylgeranyl diphosphate to the sulphur atom in the Cys residue within the CAAX sequence. S. cerevisiae proteins such as the Ras superfamily proteins Rho1, Rho2, Rsr1/Bud1 and Cdc42 appear to be GGPTase substrates (Madaule et al. (1987) PNAS 84:779-783; Bender et al. (1989) PNAS 86:9976-9980; and Johnson et al. (1990) J Cell Biol 111:143-152).
Members of the family of phospholipid-dependent, serine/threonine-specific protein kinases known collectively as protein kinase C (PKC) respond to extracellular signals that act through receptor-mediated hydrolysis of phosphatidylinositol-4,5-bisphosphate to diacyl-glycerol (DAG) and inositol-1,4,5-trisphosphate (IP3) (Hokin (1985) Annu. Rev. Biochem. 54, 205-235.). DAG serves as a second messenger to activate PKC (Takai et al. (1979) Biochem. Biophys. Res. Commun. 91:1218-1224; Kishimoto et al (1980) J. Biol. Chem. 255:2273-2276; Nishizuka (1986) Science 233:305-312; and Nishizuka (1988) Nature 334:661-665), and IP3 functions to mobilize Ca2+ from intracellular stores (Berridge et al (1984) Nature 312:215-321). Twelve distinct subtypes of mammalian PKC have been reported to date (Nishizuka (1992) Science 258:607-614; Decker et al. (1994) TIBS 19:73-77). The four initially identified isozymes, xcex1, xcex2I, xcex2II, and xcex3, are structurally closely related to each other and display similar catalytic properties.
Mammalian PKC is thought to play a pivotal role in the regulation of a host of cellular functions through its activation by growth factors and other agonists. These functions include cell growth and proliferation, release of various hormones, and control of ion conductance channels. Indirect evidence suggests that PKC induces the transcription of a wide array of genes, including the proto-oncogenes c-myc, c-fos, and c-sis, human collagenase, metallothionein IIA, and the SV40 early genes.
The PKC1 gene of budding yeast encodes a homolog of the xcex1, xcex2, and xcex3 isoforms of mammalian Protein Kinase C that regulates a MAPK-activation pathway. Loss of PKC1 function results in a cell lysis defect that is due to a deficiency in cell wall construction.
The present invention provides drug screening assays for identifying pharmaceutically effective compounds that specifically inhibit the biological activity of fungal GTPase proteins, particularly GTPases involved in cell wall integrity, hyphael formation and other cell functions critical to pathogenesis. Briefly, as described in greater detail below, Applicants have discovered the critical involvement of Rho-like GTPase activities in cell wall integrity. For instance, the fungal Rho1 GTPase is required for glucan synthase activity, copurifies with 1,3-xcex2-glucan synthase, and is found to associate with the Gsc1/Fks1 subunit of this complex in vivo. Rho1 is an regulatory subunit of 1,3-xcex2-glucan synthase, and accordingly this interaction, and the resulting enzyme complex, are potential therapeutic targets for development of antifungal agents. Moreover, Rho1 is required for protein kinase C (PKC1) mediated MAPK activation, and confers upon PKC1 the ability to be stimulated by phosphatidylserine (PS), indicating that Rho1 controls signal transmission through PKC1. Loss of PKC1 activity results in cell lysis. Also, we demonstrate that prenylation of Rho1 by a geranylgeranyl transferase is a critical step to maintenance of cell wall integrity in yeast. As described in the appended examples, prenylation of Rho1 is required for sufficient glucan synthase activity. Loss of Rho1 prenylation results in cell lysis. In general, a salient feature of the subject assays is that the each is generated to detect agent which are potentially cytotoxic to a fungal cell, rather than merely cytostatic. Moreover, given the uniqueness of the therapeutic fungal targets of the present assays, e.g., relative to homolgous proteins in mammalian cells, the therapeutic targeting of Rho-like GTPase(s) involvement in such interactions and complexes in yeast presents an opportunity to define antifungal agents which are highly selective for yeast cells relative to mammalian cells.
In one aspect, the present invention provides an assay for identifying potential anti-fungal agents by targeting the GGPTase/GTPase interaction. For instance, the assay can be run by forming a reaction mixture including (i) a fungal geranylgeranyl transferase (GGPTase), (ii) a substrate for the GGPTase, such as a target polypeptide comprising a fungal Rho-like GTPase such as Rho1, Rho2, Rsr1/Bud1 and Cdc42, or a polypeptide portion thereof including at least one of (a) a prenylation site which can be enzymatically prenylated by the GGPTase, or (b) a GGPTase binding sequence which specifically binds the GGPTase, and (iii) a test compound. The interaction of the target polypeptide with the GGPTase can be detected. A statistically significant decrease in the interaction of the target polypeptide and GGPTase in the presence of the test compound, relative to the level of interaction in the absence of the test compound (or other control), indicates a potential anti-fungal activity for the test compound.
The reaction mixture can be a reconstituted protein mixture, a cell lysate or a whole cell. For instance, the reaction mixture can be a prenylation system including an activated geranylgeranyl group, and the step of detecting the interaction of the target polypeptide with the GGPTase includes detecting conjugation of the geranylgeranyl group to the target polypeptide. In preferred embodiments of such prenylation systems at least one of the geranylgeranyl group and the target polypeptide has a detectable label, and the level of geranylgeranyl group conjugated to the target polypeptide is quantified by detecting the label in at least one of the target polypeptide, free geranylgeranyl groups, and geranylgeranyl-conjugated target polypeptide. As illustrated below, the substrate target can incorporate a fluorescent (or other) label, the fluorescent characterization of which is altered by the level of prenylation of the substrate target, e.g., the substrate target can be a dansylated peptide substrate of the fungal GGPTase.
In other embodiments, the step of detecting the interaction of the target polypeptide with the GGPTase includes detecting the formation of protein-protein complexes including the target polypeptide with the GGPTase. For example, at least one of the GGPTase and the target polypeptide can include a detectable label, and the level of GGPTase/target polypeptide complexes formed in the reaction mixture is quantified by detecting the label in at least one of the target polypeptide, the GGPTase, and GGPTase/target polypeptide complexes. Exemplary labels for such embodiments, and for the prenylation assays above, include radioisotopes, fluorescent compounds, enzymes, and enzyme co-factors. For instance, the detectable label can be a protein having a measurable activity, and one of the PKC or GTPase is fusion protein including the detectable label. In other exemplary embodiments, conjugation of the geranylgeranyl group to the target polypeptide is detected by an immunoassay.
Where the reaction mixture is a whole cell, the cell will preferably include heterologous nucleic acid recombinantly expressing one or more of the fungal GGPTase subunits and target polypeptide. In certain preferred embodiments, the cell will also include a heterologous reporter gene construct having a reporter gene in operable linkage with a transcriptional regulatory sequence sensitive to intracellular signals transduced by interaction of the target polypeptide and GGPTase.
In one preferred embodiment, the assay includes forming a cell-free reaction mixture including: (i) a fungal GGPTase, (ii) a GGPTase substrate, e.g., a target polypeptide comprising a fungal Rho-like GTPase, or a polypeptide portion thereof including a prenylation site, (iii) an activated geranylgeranyl group, (iv) a divalent cation, and (v) a test compound. The assay is derived to detect conjugation of the gernaylgernayl group of the target polypeptide in the reaction mixture, and a statistically significant decrease in the prenylation of the target polypeptide and GGPTase in the presence of the test compound, relative to an appropriate control, indicates a potential anti-fungal activity for the test compound.
In another preferred embodiment, the method utilizes an interaction trap system including (a) a first fusion protein comprising at least a portion of a fungal GGPTase subunit, (b) a second fusion protein comprising at least a portion of a fungal GTPase, and (c) a reporter gene, including a transcriptional regulatory sequence sensitive to interactions between the GGPTase portion of the first fusion protein and the GTPase portion of the second polypeptide. After contacting the interaction trap system with a candidate agent the level of expression of a reporter gene is measured and compared to the level of expression in the absence of the candidate agent. A decrease in the level of expression of the reporter gene in the presence of the candidate agent is indicative of an agent that inhibits interaction of the GGPTase and GTPase.
In still another embodiment, the assay is derived from a a recombinant cell expressing a recombinant form of one or more of a fungal GGPTase and a fungal Rho-like GTPase. The cell is contacted with a test compound, and the level of interaction of the GGPTase and Rho-like GTPase is detected. A statistically significant change in the level of interaction of the GGPTase and Rho-like GTPase is indicative of an agent that modulates the interaction of those two proteins. In preferred embodiments, one or both of a GGPTase subunit or the Rho-like GTPase are fusion proteins, e.g., the fustion protein providing a detectable label and/or an affinity tag for purification. In a preferred embodiment, the Rho-like GTPase is a fusion protein further comprising a transcriptional regulatory protein, and level of prenylation of the Rho-like GTPase is detected by measuring the level of expression of a reporter gene construct which is sensitive to the transcriptional regulatory protein portion of the fusion protein, wherein inhibition of prenylation of the fusion protein results in loss of membrane partitioning of the fusion protein and increases expression of the reporter gene construct.
In other preferred embodiments, the level of interaction of the GGPTase and Rho-like GTPase is detected by detecting prenylation of the Rho-like GTPase.
In yet another preferred embodiment, the assay is generated from a set of cells in which prenylation of endogenous Rho-like GTPases by GGPTase I is made dispensible. According to this embodiment, the assay provides a first test cell in which one or more Rho-like GPTases are mutated to be a substrate for a farnesyl transferase expressed by the cell such that GGPTase I is dispensible for cell growth; and a second test cell identical to the first cell except that the Rho-like GTPases are substrates for GGPTase I and are indispensible for cell growth. The first and second cells are contacted with a candidate agent, and the level of prenylation of the Rho-like GTPases in first and second test cells are compared. A statistically significant decrease in the prenylation of the GTPases in the second test cell, relative to the level of prenylation of the GTPase in the first cell, is indicative of an agent that inhibits interaction of a GGPTase and GTPase.
Yet another aspect of the present invention, the subject assays are derived for detecting agents which disrupt the formation of, or function of fungal protein complexes including Rho-like GTPases and PKC proteins. In one embodiment, the assay provides a reaction mixture including a fungal Rho-like GTPase, a fungal protein kinase C (PKC), and a test compound. Interaction of the Rho-like GTPase and PKC is detected in the reaction mixture, wherein a statistically significant decrease in the interaction of the Rho-like GTPase and PKC in the presence of the test compound, relative to the level of interaction in the absence of the test compound, indicates a potential antifungal activity for the test compound.
The reaction mixture can be a reconstituted protein mixture, a cell lysate or a whole cell. In preferred embodiments, the reaction mixture is a kinase system including ATP and a PKC substrate, and the step of detecting interaction of the GTPase and PKC includes detecting phosphorylation of the PKC substrate by a PKC/GTPase complex. Preferably, at least one of the PKC substrate and ATP includes a detectable label, and the level of phosphorylation of the PKC substrate is quantified by detecting the label in at least one of the phosphorylated PKC substrate or ATP. For instance, the PKC substrate may include a fluorescent (or other) label, the fluorescent characterization of which is altered by the level of phosphorylation of the PKC substrate.
In other preferred embodiments, the step of detecting the interaction of the GTPase with the PKC includes detecting the formation of protein-protein complexes including the GTPase and PKC. For instance, at least one of the PKC and GTPase includes a detectable label, and the level of PKC/GTPase complexes formed in the reaction mixture is quantified by detecting the label in at least one of the GTPase, the PKC, and PKC/GTPase complexes. For instance, phosphorylation of the PKC substrate is detected by immunoassay.
Cell-based assays are also provided, including cells comprising reporter gene constructs sensitive to PKC/GTPase complexes. In one embodiment, PKC/GTPases interaction trap assays are used for drug screening according to the present invention.
In still another aspect of the present invention, the subject assays are derived for detecting agents which disrupt the formation of, or function of fungal protein complexes including Rho-like GTPases and glucan synthase complexes or subunits thereof. In a preferred embodiment, the assay includes forming a reaction mixture including a fungal Rho-like GTPase, a fungal glucan synthase complex or subunit thereof (collectively xe2x80x9cGS proteinxe2x80x9d), and a test compound. The interaction of the Rho-like GTPase and GS protein can be detected in the reaction mixture. Similar to the assay embodiments set out above, a statistically significant decrease in the interaction of the Rho-like GTPase and GS protein in the presence of the test compound, relative to the level of interaction in the absence of the test compound, indicates a potential antifungal activity for the test compound.
The reaction mixture can be a reconstituted protein mixture, a cell lysate or a whole cell. In preferred embodiments, the reaction mixture is a glucan synthesis system including a GTP and a UDP-glucose, and the step of detecting interaction of the GTPase and GS protein includes detecting formation of glucan polymers in the reaction mixture, e.g., the UDP-glucose can include a detectable label, and the level of glucan polymer formation is quantified by detecting the labeled glucan polymers.
In other embodiments, the step of detecting the interaction of the GTPase with the GS protein includes detecting the formation of protein-protein complexes including the GTPase and GS protein. As above, at least one of the GS protein and GTPase can include a detectable label, and the level of GS protein/GTPase complexes formed in the reaction mixture is quantified by detecting the label in at least one of the GTPase, the GS protein, and GS protein/GTPase complexes. Alternatively, the formation of protein-protein complexes including the GTPase and GS protein is detected by an immunoassay.
As above, cell-based assays are also provided, including cells comprising reporter gene constructs sensitive to GS/GTPase complexes. Permeabilization of cells due to disruption of GS activity by the test compound can also be detected by loss of cytoplasmic localization or cytoplasmic exclusion (depending on the embodiment) of a detectable label.
For each of the assay embodiments set out above, the assay is preferably repeated for a variegated library of at least 100 different test compounds, though preferably libraries of at least 103, 105, 107, and 109 compounds are tested. The test compound can be, for example, small organic molecules, and/or natural product extracts.
Also, in preferred embodiments of the subject assay, one or more of the GTPase of other proteins which interacting with the GTPase (e.g., GGPTase subunits, PKC and glucan synthase subunits) are derived from a human pathogen which is implicated in mycotic infection.
The subject assay also preferably includes a further step of preparing a pharmaceutical preparation of one or more compounds identified as having potential antifungal activity.
Still another aspect of the invention concerns various compositions and reagents for performing the subject drug screening assays. For instance, the present invention provides a variety of recombinant cells expressing one or more different fungal proteins implicated as targets in the subject screening assays. In a preferred embodiment, the recombinant cell includes exogenous nucleic acid (e.g., expression vectors) encoding a fungal Rho-like GTPase. In a more preferred embodiment, the recombinant cell includes (i) exogenous nucleic acid(s) encoding one or more subunits of a fungal geranylgeranyl protein transferase (GGPTase), and (ii) exogenous nucleic acid encoding a fungal Rho-like GTPase or a fragment thereof including at least one of (a) a prenylation site which can be enzymatically prenylated by the GGPTase, or (b) a GGPTase binding sequence which specifically binds the GGPTase. In still other preferred embodiments, the cell includes (i) exogenous nucleic acid encoding a fungal Rho-like GTPase, and (ii) exogenous nucleic acid encoding a fungal protein selected from the group consisting of a fungal protein kinase C (PKC) or one or more subunits of a fungal glucan synthase.
The nucleic acids encoding the GGPTase, GTPase, PKC and/or glucan synthase are preferably derived from a human pathogen which is implicated in mycotic infection. For instance, the recombinant genes can be derived from fungus involved in such mycotic infections as selected from a group consisting of candidiasis, aspergillosis, mucormycosis, blastomycosis, geotrichosis, cryptococcosis, chromoblastomycosis, penicilliosis, conidiosporosis, nocaidiosis, coccidioidomycosis, histoplasmosis, maduromycosis, rhinosporidosis, monoliasis, para-actinomycosis, and sporotrichosis. To further illustrate, the expression vectors can be generated from genes cloned from human pathogen selected from a group consisting of Candida albicans, Candida stellatoidea, Candida tropicalis, Candida parapsilosis, Candida krusei, Candida pseudotropicalis, Candida quillermondii, Candida rugosa, Aspergillus fumigatus, Aspergillus flavus, Aspergillus niger, Aspergillus nidulans, Aspergillus terreus, Rhizopus arrhizus, Rhizopus oryzae, Absidia corymbifera, Absidia ramosa, and Mucor pusillus. Another source for recombinant genes is the human pathogen is Pneumocystis carinii. 
In preferred embodiments, the cell is a recombinantly manipulated yeast cell selected from the group consisting of such genuses as Kluyverei, Schizosaccharomyces, Ustilaqo and Saccharomyces, though a preferred host cell is the Schizosaccharomyces cerivisae cell. Moreover, the host cell can be constitutively or inducibly defective for an endogenous activity corresponding to one or more of the GGPTase and GTPase encoded by the exogenous nucleic acids.
In similar fashion, another aspect of the present invention concerns reconstituted protein mixtures or cell lysate mixtures including a recombinant fungal Rho-like GTPase, e.g, or a fragment thereof including at least one of (a) a prenylation site which can be enzymatically prenylated by the GGPTase, or (b) a GGPTase binding sequence which specifically binds the GGPTase, along with one or more of a recombinant fungal glucan synthase, a recombinant fungal GGPTase, and/or a recombinant fungal PKC. As above, the fungal target proteins are preferably derived from a human pathogen which is implicated in mycotic infection.
Another aspect of the present invention relates to the discovery and isolation of genes encoding novel regulatory proteins from the human fungal pathogen Candida, namely the xcex1 and xcex2 subunits of a GGPTase I enzyme and several Rho-like GTPases. The present invention specifically contemplates a purified and/or recombinant polypeptide including a GTPase sequence encodable by a nucleic acid which hybridizes under stringent conditions to SEQ ID No. 1, a Candida CaRho1 gene, or to SEQ ID No. 5, a Candida CaCdc42 gene, the GTPase sequence (i) directing the binding of the polypeptide to a glucan synthase subunit, (ii) directing the binding of the polypeptide to PKC, (iii) serving as a substrate for prenylation by a GGPTase, or (iv) having a GTP hydrolytic activity, or a combination thereof. In other embodiments, there is provided a purified and/or recombinant polypeptide including a Cal1 or RAM2 sequence encodable by a nucleic acid which hybridizes under stringent conditions to SEQ ID No. 3 or 7, a Candida CaRAM2 or CaCal1 gene, the RAM2 or Cal1 sequence (i) directing the binding of the polypeptide to other GGPTase subunits, or (ii) directing the binding of the polypeptide to a Rho1-like GTPase, or a combination thereof.
In preferred embodiments of the above polypeptides, the GTPase sequence or the GTPase subunit sequence is at least 80% identical, more preferably 90% identical, and even more preferably identical to one of the polypeptides represented by SEQ ID Nos. 2, 4, 6 or 8.
The subject polypeptides can be derived from, e.g., encoded by, an endogenous gene from Candida spp. Exemplary Candida organisms include Candida albicans, Candida stellatoidea, Candida tropicalis, Candida parapsilosis, Candida krusei, Candida pseudotropicalis, Candida quillermondii and Candida rugosa. 
In some embodiments, the polypeptide is a fusion protein. For instance, the fusion protein can include, in addition to the GTPase subunit sequence, a RAM1 or cdc43 sequence, the fusion protein possessing prenylation activity. In other embodiments, the fusion protein can include, in addition to the GTPase or GTPase subunit sequence, as appropriate, a second polypeptide portion selected from the group consisting of a DNA binding domain and a transcriptional activitation domain, the fusion protein being functional in a two-hybrid assay.
Still another aspect of the present invention relates to purified protein complexes including the GTPase or RAM2 polypeptide described herein. For instance, in the case of the complexes including CaRho1 or CaCdc42, the protein complex can also include a glucan synthase subunit, a PKC, a GGPTase, or a combination thereof. Exemplary complexes including the subject CaRAM2 polypeptide include a GGPTase xcex2 subunit, an FPTase xcex2 subunit, a Rho1-like GTPase, or a combination thereof.
Yet another aspect of the present invention relates to isolated nucleic acids including a coding sequence encoding one of the subject CaRho1, CaCdc42, CaCal1 or CaRAM2 polypeptides. The present invention also provides isolated nucleic acids which specifically hybridizes to the nucleic acid sequence of SEQ ID No. 1, 3, 5 or 7 (sense or antisense) and which selectively detect either a CaRho1 or CaCdc42 gene (e.g., encoding a protein having GTP hydrolytic activity) or a CaCal1 or CaRAM2 gene (e.g., encoding a GGPTase or FPTase subunit).
Such nucleic acids can be provided as part of a diagnostic test kit for detecting Candida cells. For instance, the nucleic acid can be an antisense oligonucleotide which hybridizes to CaRho1, CaCdc42, CaCal1 or CaRAM2 gene, as appropriate. In preferred embodiments, the nucleic acid is at least 25 nucleotides in length, though more preferably at least 50 nucleotides in length.
In many embodiments of the kit, the nucleic acid will be labelled with a detectable label. Exemplary detectable labels include enzymes, enzyme substrates, coenzymes, enzyme inhibitors, fluorescent markers, chromophores, luminescent markers, and radioisotopes.
In preferred embodiments, the kit, by selection of the nucleic acid, is designed to detect the presence of nucleic acid from a Candida cell selected from the group consisting of Candida albicans, Candida stellatoidea, Candida tropicalis, Candida parapsilosis, Candida krusei, Candida pseudotropicalis, Candida quillermondii and Candida rugosa. 
The present invention also provides expression constructs encoding the subject CaRho1, CaCdc42, CaCal1 or CaRAM2 proteins, as well as host cells transformed with such expression constructs. Furthermore, the present invention provides a method for producing a recombinant CaRho1, CaCdc42, CaCal1 or CaRAM2 polypeptide by culturing such host cells under conditions sufficient to produce a cell culture expressing the polypeptide, and isolating the polypeptide from the cell culture.
Still another aspect of the present invention provides an isolated, recombinant and/or monoclonal antibody which is specifically cross-reactive with the subject CaRho1, CaCdc42, CaCal1 or CaRAM2 proteins. The antibody can be labelled with a detectable label, such as enumerated above.
The practice of the present invention will employ, unless otherwise indicated, conventional techniques of cell biology, cell culture, molecular biology, transgenic biology, microbiology, recombinant DNA, and immunology, which are within the skill of the art. Such techniques are explained fully in the literature. See, for example, Molecular Cloning: a Laboratory Manual, 2nd Ed., ed. by Sambrook, Fritsch and Maniatis (Cold Spring Harbor Laboratory Press: 1989); DNA Cloning, Volumes I and II (D. N. Glover ed., 1985); Oligonucleotide Synthesis (M. J. Gait ed., 1984); Mullis et al. U.S. Pat. No: 4,683,195; Nucleic Acid Hybridization (B. D. Hames and S. J. Higgins eds. 1984); Transcription And Translation (B. D. Hames and S. J. Higgins eds. 1984); Culture of Animal Cells (R. I. Freshney, Alan R. Liss, Inc., 1987); Immobilized Cells And Enzymes (IRL Press, 1986); B. Perbal, a Practical Guide To Molecular Cloning (1984); the treatise, Methods In Enzymology (Academic Press, Inc., N.Y.); Gene Transfer Vectors For Mammalian Cells (J. H. Miller and M. P. Calos eds., 1987, Cold Spring Harbor Laboratory); Methods In Enzymology, Vols. 154 and 155 (Wu et al. eds.), Immunochemical Methods In Cell And Molecular Biology (Mayer and Walker, eds., Academic Press, London, 1987); Handbook of Experimental Immunology, Volumes I-IV (D. M. Weir and C. C. Blackwell, eds., 1986); Manipulating the Mouse Embryo, (Cold Spring Harbor Laboratory Press, Cold Spring Harbor, N.Y., 1986).